Tunable coupler for electrically short monopole antennas



P 1970 v P. J. KHAN ET AL 3,508,272

' TUNABLE COUPLER FpR ELECTkICALLY SHORT MONOPOLE ANTENNAS Filed July29, '1968 OUT FIG. 3 (a) I FIG. 30:)

[BAND l- Me. 5 5 1 0 1 5 2 0 2Y5 s o 3': 4'0

CIR cu IT 1 i RjSONATE 'S CIRCUIT 2 RESONATES MONOPOLE I ANTENNAINVENTORS,

PETER J. KHAN GARY A. VANDER HAAGEN DAVID E. OLIVER. BY Mihzn. W, AGENIW 12 *wjh/ ATTOK'NEY3 United States Patent US. Cl. 343-745 4 ClaimsABSTRACT OF THE DISCLOSURE A wide-range electronically tunable antennacoupling system for use with electrically short monopole antennae.Impedance matching is realized by the use of a unique varactor diodecircuit which provides tunable impedance matching over a very wide rangeof frequencies.

BACKGROUND OF THE INVENTION This invention relates to coupling systemsfor antennae and more particularly to a wide-range,electronicallytunable power matching antenna coupling system for usewith electrically short monopole antennae.

Prior art attempts to provide wide range tuning and impedance matchingbetween an electrically short monopole antenna and a receiver haveconsisted primarily of electromechanical switching techniques, biasedferrite antenna networks or mechanical tuning circuits. Conventionalantenna coupling circuits have been severely limited in their operatingfrequency range due to the essential requirement of circuit gain andreliability which could not be sacrificed in lieu of a broad operatingfrequency range. At best, such circuits were capable of tuning over a2.5/1 frequency range.

The present invention overcomes the disadvantages of the prior art bythe utilization of a unique varactor diode circuit exhibiting wide rangeelectrical tuning capability in excess of a 6 to 1 frequency rangeratio, while simultaneously maintaining the desired circuit gain,simplicity and reliability.

The general purpose of this invention is to provide a wide-range,electrically-tunable, power-matching, receiving antenna coupling systemfor use with electrically short monopole antennae.

BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of this inventionwill be readily apparent from consideration of the followingspecification relating to the annexed drawings wherein:

FIGURE 1 shows an equivalent circuit for a short monopole antenna;

FIGURE 2 shows an L-type matching network for the equivalent circuit ofthe monopole antenna shown in FIGURE 1;

FIGURE 3(1)) shows a frequency diagram of the coupling circuit shown inFIGURE 3(a) and FIGURE 4 shows one arrangement of a monopole antennamatching network as envisioned by the instant disclosure.

DESCRIPTION OF THE INVENTION In order to render the theoretical aspectsof the coupling network more discernible, a basic equivalent circuit ofa short monopole antenna is shown in FIGURE 1. Looking back into theoutput terminals of a monopole an tenna, as represented by terminals and11 of FIGURE 1, a capacitive reactance 12 would be presented in serieswith a parallel arrangement of an inductive reactance 13 and aresistance 14.

As seen in FIGURE 1, an electrically short monopole antenna ischaracterized by a highly capacitive reactive impedance, such that thedirect connection of such an antenna to a coaxial cable for transmissionof an input signal to a receiver would result in a large mismatch orsignal loss. This problem of mismatch was eliminated by the use of avariable reactance coupling network schematically shown in its simplestform in FIGURE 2.

The basic coupler 20, shown in FIGURE 2, is essentially an inductivetype matching network including a variable series inductor 15, which isadjustable to such a value as to produce a series resonant circuit withthe antenna capacitance 12. A variable capacitive device 16, is chosento be parallel-resonated with antenna inductance 13. At resonance, theresistance appearing between terminals 17 and 18 is the antennaresistance 14. A broadband transformer 19 having a resistivecharacteristic equal to the antenna resistance at resonance is connectedacross terminals 17 and 18 as a terminating load device. The primarywinding of broadband transformer 19 is matched to the coupler impedanceat resonance and the secondary winding of the transformer matches theimpedance of the coaxial transmission line connecting the coupler to thereceiving unit.

Monopole antennas are generally required to operate over a wide range offrequencies, therefore, the inductive and capacitive elements of thecoupler must be tunable by a ratio equal to or greater than (Af) whereVariable inductive and capacitive elements currently available areeither conventional mechanical devices or saturable reactors andvaractor diodes.

As a typical example of the invention, a 5 to 30 me. antenna couplerwith tuning provisions was developed. The circuit features which bestrepresent the spirit of the invention will be contained therein. Theprocedure employed in the development of a variable inductance forresonating with the monopole capacitance was as follows, with thecircuit shown in FIGURE 3(a). The capacitors 30 and 40 were chosen to bevaractors having a capacitance range of 10 pf. to pf. The inductiveelement 31 was chosen to resonate at a maximum value of capacitance 30at a frequency just above 5 mc., here chosen to be 5.4 mc. Inductance 41was chosen to resonate with the maximum value of capacitance 40 at 10.4-me. The 5 me. to 30 me. frequency range was divided into two bands shownin FIGURE 3'(b), such that inductance 31 and capacitance 30 principallydetermine the impedance on band 1, and inductance 41 and capacitance 40principally determine the band 2 impedance.

The variable capacitor 40 was adjusted to have a value of 120 pf. forthe tuning of frequencies in band 1, and the exact value of inductancerequired was obtained by variation of capacitor 30. Capacitor 30 wasmaintained at a value of 120 pf. for the tuning of band 2 frequencies.The procedure provided a guide to the adjustment of capacitors 30- and40', thus providing a basis for the choice of varactors, to give theappropriate inductance over the frequency range. However, more detailedcomputation was required to determine the capacitance values ofcapacitors 30 and 40 at some frequencies in band 1 and the lower end ofband 2. The adjustment was carried out experimentally. Therefore, thevalues of capacitors 30 and 40 were adjusted such that at the frequencyof operation, the two circuits were operating at a point below selfresonance, at which point the inductive reactance of the circuit equaledthe capacitive reactance of the antenna. The amount of inductivereactance was controlled by the deviation from resonance.

Referring now to FIGURE 4, wherein a complete antenna coupler issymbolically shown, the two resonant circuits developed above and shown.in FIGURE 3(a) are used in conjunction with a third resonant circuitwhich appears capacitive to the monopole antenna and thus matches theinductive reactance of the antenna for extending the efiective bandwidthof the coupler as evidenced by FIGURE 3(1)).

The variable capacitive device 16 shown in FIGURE 2 may be replaced withthe variable resonant circuit of FIGURE 4 which comprises an inductivedevice 52 in series with a variable capacitor 51 having a secondvariable capacitor 50 connected in a parallel arrangement with theseries inductance 52 and capacitor 51. A similar procedure to thatoutlined above for the series resonant circuits may be used fordetermining the value and type of elements required in the parallelresonant circuit. Here, however, operation of the series inductor 52 andcapacitor 51 combination is operated above resonance such that theparallel resonant circuit of capacitor 50 and inductor 52 with capacitor51, always appears capacitive. In actual practice, the variablecapacitive devices 50 and 51 are replaced with a pair of varactorsexhibiting the desired characteristics. The net variation in inductanceand capacitance of the parallel capacitive arrangement, realized byutilizing the above described technique, approaches the product of theratios of the change in capacitance of the two varactor diodes employed.For example, if the ratios of the two varactors used are approximately 3to 1 and 2 to 1 respectively, the total range would be the product ofthe two ratios or approximately 6 to 1.

This technique allows multiple diodes to act as either variablecapacitors or inductors at low loss with the realization of wide rangeelectrical tuning capability in excess of a 6 to 1 frequency range.

The coupling network herein described may be used in various radioreceiving systems and is in no way limited to the use described in thespecification. It should be understood that the coupler may take manymounting forms which would suggest to those skilled in the art variousalternative supporting and coupling arrangements.

We claim:

1. A wide-range electronically turnable antenna coupling network forcoupling an electrically short monopole antenna to a load terminationdevice, comprising:

first and second parallel resonant circuits connected in series betweenone terminal of a two-terminal monopole antenna and one terminal of atwoterminal load termination device, said resonant circuits exhibitingan inductive reactance which is equal 4 to the absolute value of thecharacterstic capacitive reactance of the monopole antenna when operatedat a frequency slightly below self resonance; and

a third parallel resonant circuit connected across the terminals of theload termination device and in parallel therewith;

one terminal of said load termination device provides a commonconnecting point be'twen the series connected parallel resonant circuitsand the parallel connected parallel resonant circuit;

a second terminal of said load termination device provides a commonconnecting point for the other terminal of said parallel connectedresonant circuit and the other terminal of said monopole antenna, saidthird resonant circuit exhibiting a capacitive reactance which is equalto the absolute value of the characteristic inductive reactance of themonopole antenna when operated at a frequency slightly above selfresonance;

whereby the coupler exhibits an output impedance equal to thecharacteristic resistance of the monopole antenna.

2. The coupling network as set forth in claim 1, wherein said thirdresonant circuit comprises a first varactor in parallel arrangement witha series combination of a second varactor and an inductive element.

3. The coupling network as set forth in claim 1, wherein each of saidfirst and second parallel resonant circuits consists of a parallelarrangement of a varactor and an inductive element.

4. The coupling network as set forth in claim 3, wherein said thirdresonant circuit comprises a first varactor in parallel arrangement witha series combination of a second varactor and an inductive element.

References Cited UNITED STATES PATENTS 2,745,067 5/1956 True et a1.33317 2,920,323 1/1960 Dunson 343--850 3,098,231 7/1963 Vrain et a1.343-745 3,110,004 11/1963 Pope 307-320 XR ELI LIEBERMAN, PrimaryExaminer M. NUSSBAUM, Assistant Examiner US. Cl. X.R. 343-861

